FIELD OF THE INVENTION
The present invention relates to braking systems for vehicles, and in particular to electronic braking systems (EBS) which have a back-up braking facility. The invention is particularly suited to trailer braking systems for towing vehicles.
EP 0250738A (Bosch) discloses a method of checking that the level of pressure in the main circuit is consistent with both the drivers demand and the auxiliary backup pressure.
On every brake application, once the drivers demand exceeds a predetermined pressure level of 4 bar, the electronically controlled main circuit is disabled and the auxiliary backup circuit is enabled. A pressure sensor located downstream of the two valves (5) and (6) measures the differential between the main circuit and auxiliary circuit at the changeover point, and if the differential exceeds a predetermined level then the controller flags un an error.
With increasing demand the auxiliary circuit remains in operation and is directly connected to the brake. It is stated that this is because the auxiliary circuit pressure can be controlled more accurately with the solenoid valve 6. This is because once the pressure has risen within the actuator level where all the `slack` has been removed, only a small increase in additional volume is required to achieve a corresponding larger increase in pressure. Therefore, valve 6, must be a `fine control` solenoid valve, only capable of small volume flow rates. Bearing this in mind, if a fault developed with either valve (5) or (7) and the main circuit was disabled permanently, the backup auxiliary system operating from the foot valve (9) via the fine control, valve (6) would be responsible for the total brake control. This would mean that a brake application would be extremely slow, the full brake volume now having to pass through the valve (6), additionally a brake release would be equally as slow, thus holding the brake in the applied condition when the drivers demand is removed.
In the arrangements as shown several major faults exist which mean that in the event of the main circuit failure even with the introduction of the auxiliary backup circuit, the brake itself cannot be applied. Referring to FIG. 1 in particular, if the exhaust valve (7) seized in the open condition thus permanently opening the brake actuator to atmosphere, the application of pressure via the `fine control` valve (6) would simply be exhausted to atmosphere, having had no apparent effect on the brake actuator.
Thus, although EP 0250738 shows that a backup auxiliary pressure can be introduced into a service brake actuator via a solenoid valve, it does not show how to achieve correct application of the backup auxiliary pressure when a fault exists in the main circuit system.
In EP 0345203A (Daimler-Benz), an attempt has been made to isolate the backup auxiliary system from the main service line to prevent discharge of the auxiliary backup system via any fault developed in the main service control channel. However, in an attempting to achieve the above other areas of concern have not been attended to.
Normally, a failure in the service control channel would take the form of a sticking or leaking inlet or exhaust valve (5) or (6). If this occurs, it must be ensured that the auxiliary system can be switched on and it must be Guaranteed that once employed the backup system cannot be disengaged by any fault in the service control line.
In the Daimler-Benz arrangement, if a fault as above developed, i.e., inlet valve (5) developed a leak or seized in the open position, then, even if the exhaust valve was open, pressure in the small pilot line 7.2 would develop to a sufficient enough level, 0.02 bar, to cause the 3/2 valve to switch into the `working` position, therefore isolating the backup system, just when required, during an inlet valve failure.
If a corresponding fault developed in the exhaust valve (6) for example, if the valve seized in the closed position, the pressure in the service line together with pressure in the pilot line could not be exhausted to atmosphere. Therefore, the pilot valve would remain latched and the backup system would be isolated from the brake actuator, at again, the very instant it would be required.
EP 0345203 also shows embodiments where within the pilot line 7.2 a 2/2 flow control valve is placed. This valve is solely used for `desensitising` the pilot valve from ABS operation, where upon an antilock brake dump the pressure in the (5) and therefore also line (7.2) is exhausted to atmosphere, this would cause the 3/2 pilot valve to unlatch and bring in the auxiliary backup pressure. Therefore a high demand pressure would be fed directly from the foot valve (9) to the brake actuator (3) just at the moment when the ABS system is trying to remove all pressure from the actuator. To cope with this situation a restriction is placed in the pilot line in the `off` direction so as to prevent the rapid decay of pilot pressure and therefore unlatching of the 3/2 valve.
Unfortunately, due to the fact that by their very nature, pilot lines contain very small volumes, even with a severe restriction in 14 ne (7-2) the pilot pressure will still decay very quickly, therefore on a skid cycle of long duration i.e. low m.mu. surface such as ice or snow, the 2/2 position valve as shown, even if it is solenoid controlled, will not prevent the auxiliary back-up system from being deployed, causing a total lockup of the brake in question.
Although EP 0345203 does teach us to attempt to isolate the main service line pressure from the auxiliary backup system, it does not teach use, as with EP 0250738 Bosch, how to prevent the backup from being disabled by a fault in the main service line control system.
DE 4004502 (Daimler-Benz) is a development of EP 0345203A, in which the problem of coping with a long skid cycle as discussed above has been realised. A solution is proposed where the 2/2 flow control valve with a large restriction in the pilot line is replaced by a 2/2 solenoid controlled isolation valve. This would indeed prevent the unlatching of the 3/2 valve on an ABS cycle. It is further stated that the 2/2 valve can only be used during ABS as it can be seen if the logic of controller (4) is followed. An ABS signal must be present at AND gate (4.2) in order to provide part of the signal at AND gate (4.1h) which switches on the drive circuitry for the 2/2 solenoid. Therefore this valve can never be used as part of some auxiliary circuit enable/disable control system for normal braking as the basic ABS signal would not be present. Bearing this in mind, it is obvious that claim 2 is unworkable as it requires the 2/2 valve to work when no skid signal is present. Apart from the above, the device of DE 4004502 is identical to that of EP 0345203 and being so, it also suffers the same failure problems.